I have a Sony Alpha100 camera and I am new to photography so please excuse my question if it seems obvious. I have taken quite a few pictures that I am satisfied with but when I take pictures of white objects, I get a blue fringe around the object. I thought it might be the lens but it does not always happen. Can someone give me an idea as to what I am doing wrong?

It's probably the lens. I think you're seeing some Chromatic Aberrations, and this often manifests itself as purple fringing on high contrast edges with digital, due to the way the light interacts with the microlenses on the sensor.

Most lenses like this are going to be a little softer on their long end (you said zoom, so I'm asssuming this is the Sony 75-300mm f/4.5-5.6), and you're shooting with the aperture wide open at f/5.6 (which is going to be it's softest aperture at that focal length).

So, you're shooting at the softest part of the lens, using a wide open aperture.

But, even the best lenses can get a bit of purple fringing with these types of shots, especially with their aperture wide open.

I'd try stopping down the aperture a bit (for example, shoot in Av mode at f/8 or f/11 instead of wide open at f/5.6 on the long end) and see if that helps.

If you've got a filter on it, you may want to try it both on and off the camera while taking some test shots when you run into similar subjects (white against a darker background) to see if any filter you're using is a contributing factor, too. Some filters have been known to degrade image quality..

Overexposure can also be a cause of purple fringing, and the bright white edge may looks like that could also been a contributing factor, since the highlights due appear to be a bit blown. When this happens (purple fringing in overexposed areas) it's referred to as blooming with digital.

But, chances are, it's just a characteristic of this lens. That's pretty common on this type of lens. I've seen a few comments in some user reviews about heavy CA (Chromatic Abberations) on it's long end, unless it's stopped down (use a smaller aperture, represented by a higher f/stop number)